Speakerphone devices, also known as hands-free telephones, are in widespread use. A conventional speakerphone contains one or more microphones and one or more speakers. The microphone converts local sound waves into electrical signals for transmission by a telephone network. The speaker converts electrical signals received from the telephone network into sound waves. There are two basic types of speakerphones: full-duplex and half-duplex. A full-duplex speakerphone user perceives the speakerphone as providing simultaneous transmission and reception, i.e., the microphone and speaker are operating simultaneously, whereas a half-duplex speakerphone user perceives the speakerphone switching between transmit and receive modes (or dramatically change the gain of the microphone and speaker paths) depending on whether the far-end party or the speakerphone user is talking the loudest. The full-duplex speakerphone relies on a hybrid (a well-known circuit that converts bidirectional two-wire communications channel (e.g., the telephone network) into a four-wire communications channel (i.e., two unidirectional communication paths) in conjunction with adaptive echo canceller or the like to suppress any acoustic feedback between the speaker and microphone that can manifest itself as “acoustic howling”. A half-duplex speakerphone is generally much simpler than a full-duplex speakerphone and relies on circuitry (including a hybrid) to determine the mode of operation depending on the level or volume of the signal received from the telephone line compared to the level of the signal from the microphone to be sent to the telephone line for transmission.
Full duplex speakerphones have the advantage of operating like a conventional telephone set: the user can speak to the other party at the same time speech or other sounds from the far-end party are being received. Half-duplex speakerphones allow communication one direction at a time and whoever talks the loudest determines the direction of communication. In some applications, high microphone gain is required to assure that the far-end party can hear sounds in the vicinity of the speakerphone, such as during an emergency condition when the speakerphone user might not be able to speak loud enough or is immobilized and is unable to get close enough for normal speakerphone operation. If sufficient ambient noise is present, the high microphone gain may sufficiently overwhelm the control circuitry in a half-duplex speakerphone that the far-end party is unable to talk loud enough to force the speakerphone into the receive mode so that the far-end party can query the speakerphone user about the nature of the emergency, her injury, needs, etc. In full-duplex speakerphones, the adaptive filtering might not be sufficient to overcome the highly amplified ambient noise, preventing the far-end party's voice from emanating from the speakerphone.